In recent years, facsimile (generally called fax) machines have become very popular for transmitting graphic information from one place to another, particularly over the public switched telephone network. A fax transmission can be made as simply as a telephone call, using a fax machine at each end of the line instead of a telephone instrument. Within each fax machine are means for producing electrical signals representative of an image to be transmitted, and for coupling these signals to a telephone system. Also within each machine is a receiver for receiving fax signals and for reconstructing an original image in printed form.
In order to facilitate fax communications between large numbers of users, an international organization, The International Telephone and Telegraph Consultative Committee (CCITT), has defined certain standards (called Recommendations) to be utilized by users of fax to communicate with other users. These standards are intended to permit communication between machines made by different manufacturers possibly located in different countries. The CCITT has adopted compatibility standards covering the transmission and reception of fax signals according to four speed/quality groups: Groups 1, 2, 3, and 4. The vast majority of fax signals today are sent and received according to the Group 3 compatibility standard (CCITT Recommendation T.4). Groups 1 and 2 are relatively slow earlier standards, while Group 4 requires a special high quality telephone line and has not yet gained popularity. The present invention is therefore described in connection with the Group 3 standard. It should be understood, however, that its applicability is not necessarily limited to Group 3 communications.
Fax transmission starts by scanning an original page in raster fashion. The variations of print density on the page are converted into a digital video signal which is subsequently coded for efficient transmission. This coded signal is modulated on an audio carrier and sent over a telephone network. At a remote terminal the signal is received, demodulated, decoded and used to cause a printing device to print an image corresponding to the original.
There are two modes of transmission defined by the CCITT Group 3 standard, standard and fine. The fine mode raster has a resolution of 7.7 lines per mm (about 196 lines per inch) vertically, with each line including 1,728 picture elements ("pels") along a standard 215 mm scan line, i.e. 204 pels per inch horizontally. Each fine mode pel therefore represents a small rectangular area about 5 mils square (1 mil=0.001 inch). The standard mode raster has half the vertical resolution of the fine mode (3.85 lines per mm), but the same horizontal resolution (i.e., 98 V.times.204 H picture elements per inch). A standard mode pel is thus about 10 mils high by about 5 mils wide. According to the CCITT Recommendation T.4, the nominal pel sizes as defined should be maintained within plus or minus one percent.
The printing mechanism of most prior art fax receivers uses a thermal printhead which is essentially a linear array of resistors, each of which produces heat in response to received electrical signals. By passing thermally sensitive paper over the printhead while received signals are causing the print elements of the printhead to be heated, dark spots corresponding to the dark picture elements of the original copy are formed. The totality of the dark spots formed during the transmission of a fax image is intended to be a copy of the original image.
An important disadvantage of the system as described above is the necessity to use special thermally sensitive paper on which the received image is formed. Not only is thermally sensitive paper expensive, but its handling and surface characteristics are such that most people find it to be undesirable. It is therefore an object of the present invention to provide a fax machine which will receive CCITT standard fax signals, but will print an image on plain paper rather than on thermally sensitive paper as used in the prior art.
Not all fax machines are of the thermal type; there are some which print on plain paper. Most of these utilize a relatively expensive laser printing engine, but some utilize an ink jet type of printhead. The ink jet type plain paper fax machines presently on the market, while less expensive than laser printer types, print more slowly than the incoming data stream, and are therefore not desirable.
In a previously issued patent, U.S. Pat. No. 4,972,270, the present inventors disclosed methods for forming fax images on plain paper using an ink jet printing cartridge which, when mounted square with respect to he scanning direction, is intended to print images with a line density of 300 lines per inch. Two methods were disclosed, one where the printhead was slanted to achieve the 98 and 196 lines per inch line densities used in Group 3 fax transmissions, and the other which approximated Group 3 line densities (actually providing 100 and 200 lines per inch) by printing dots in a certain pattern using a squarely mounted printhead.
At the time the application for the aforementioned patent was filed, only one ink jet printhead cartridge was available on the market. This printhead cartridge was manufactured by the Hewlett-Packard Company for its DeskJet series of printers and had a basic 300 lines per inch line density (i.e., the line density when the nozzle column is perpendicular to the direction of scanning). Since that time two other ink jet cartridges have appeared, but neither is intended to print with a line density in accordance with CCITT fax standards.
It is an object of the present invention to create facsimile images consistent with CCITT recommendations (in particular Recommendation T.4 for Group 3 facsimile) using ink jet cartridges which have nozzle spacings other than equal to the line spacings specified in the CCITT facsimile recommendations.
It is a further object of the present invention to utilize an ink jet type of printhead in a plain paper fax machine which prints faster than ink jet type fax machines which are presently on the market.